1. Field of the Invention
The present invention generally relates to the field of optoelectronic imaging devices, and more specifically to an optoelectronic imaging apparatus for a guided missile tracking system or the like which produces two simultaneous images of a scene or target in different optical wavelength bands.
2. Description of the Related Art
Optoelectronic imaging systems for guided missile tracking and the like are generally of the scanning or staring type. Mechanical scanning systems use motor drives to move mirrors or other scanning elements to scan a scene and sequentially focus optical images of incremental portions of the scene on a linear photodetector array. Electrical signals generated by the array are combined to construct a composite electronic image of the scene. A typical example of a scanning type optoelectronic imaging system is disclosed in "Thermal Imaging System", by J. Lloyd, Plenum Press, 1979, pp. 324-351.
Mechanical scanning systems are limited in speed, due to the inherently slow motor drives and in sensitivity due to the limited number of detectors used. For this reason, "staring" optoelectronic imaging systems have been developed in which an image of the entire scene is focussed by a Cassegrain telescope or other type of optical imaging assembly onto a rectangular focal plane photodetector array. The imaging system continuously "stares at" the entire scene, rather than scanning it. A composite image of the scene is produced by electrically scanning the photodetector elements of the array, which can be much faster than mechanical scanning. An exemplary staring type optoelectronic imaging system is disclosed in "Mitsubishi Thermal Imager Using the 512.times.512 PtSi Focal Plane Arrays", by S. Fujino et al, in Proceedings of SPIE --The International Society for Optical Engineering Infrared Technology XV, Aug. 7-9, 1989, San Diego, Calif., Vol. 1157, pp. 136-152.
Regardless of type, conventional optoelectronic imaging systems are designed to be sensitive to electromagnetic radiation in one optical wavelength band, for example visible light, medium wavelength infrared (MWIR), or long wavelength infrared (LWIR) radiation. In infrared systems especially, the photodetector array is cooled to reduce parasitic thermal noise and increase the sensitivity. The photodetector array and associated cooling apparatus are mounted in an evacuated chamber or "dewar", which occupies a relatively large portion of the extremely limited space available in a missile tracking system or the like.
In various applications, it is desirable to obtain two simultaneous images of a scene in different optical wavelength bands, such as the visible band and one of the infrared bands. In a missile system, this enables daytime tracking using the visible image, and nighttime tracking using the infrared image. It may also be desirable to obtain simultaneous MWIR and LWIR images.
"Two-color" or "dual-band" scanning systems have been constructed which include a beamsplitter to split the optical image from a telescope into two branches, and a separate photodetector array and appropriate optical bandpass filter in each branch. A system of this type is disclosed in "Conceptual Design of the High-Resolution Imaging Spectrometer (HIRIS) for EOS", by M. Hening, in Proceedings of SPIE --The International Society of Optical Engineering, Remote Sensing, April 3-4, 1986, Orlando, Fla., Vol. 644, pp. 82-85. However, such a system is too large for an application such as a missile tracker since a separate dewar is required for each photodetector array, and the optical paths for the two branches from the beamsplitter occupy an unacceptably large amount of space. In addition, the optical system requires precision alignment, which greatly increases the cost and reduces the reliability of the apparatus. Additional systems have been constructed which can image in two or more spectral bands by mechanically and sequentially inserting different spectral bands into the optical path.